1. Technical Field
The present invention generally relates to an optical communications system and, more particularly, to a fiber optic communications system for simultaneously transmitting information signals having different wavelengths over a same optical fiber.
2. Description of the Relevant Art
In recent years there has been a great deal of interest in the transmissions of various types of information television signals via optical fiber. Presently, most CATV systems for distributing television signals operate by modulating the video, audio, and other information for each television channel onto a respective radio frequency carrier signal. Each of these carrier signals typically has a bandwidth of 6 MHz. A plurality of these signals covering a broadband of radio frequencies (e.g., in the range of 54-550 MHz) are distributed via networks including 75 ohm coaxial cables and appropriate signal amplifiers and taps.
Optical fibers intrinsically have more information carrying capacity than do the coaxial cables which are used in present CATV systems. In addition, optical fibers are subject to less signal attenuation per unit length than are coaxial cables adapted for carrying radio frequency signals. Consequently, optical fibers are capable of spanning longer distances between signal regenerators or amplifiers than are coaxial cables. In addition, the dielectric nature of optical fiber eliminates the possibility of signal outages caused by electrical shorting or radio frequency pickup. Finally, optical fiber is immune to ambient electromagnetic interference ("EMI") and generates no EMI of its own.
A number of means are available for transmitting television signals and/or other types of information over optical fibers or other optical transmission media. For example, the 6 MHz baseband television signal may be converted to digital form. This digital information may be used to modulate a light signal which is transmitted via an optical link. Transmission of such a digitized 6 MHz video signal requires a digital data transmission rate of at least 45 megabits per second. High definition video ("HDTV") may require a digital data transmission rate of up to 145 megabits per second. Although emerging compression technologies may reduce these data rates, encoders and decoders for converting analog television signals to digital form and for reconverting these digital signals to analog form for viewing on a conventional television set are quite expensive. Consequently, analog transmission of television signals by optical means is, potentially, much more economical than digital transmission of such signals.
One such means of analog transmission is to use the baseband video signal to frequency modulate a radio frequency carrier. This modulated radio frequency carrier is in turn used to modulate an optical signal. Such frequency modulation is less susceptible to noise than is amplitude modulation, but it requires more bandwidth for each television channel than is required by amplitude modulation methods. Thus, the number of television channels which can be carried by each optical transmission link (e.g., each optical fiber) in an FM-based system may be somewhat limited. Moreover, since the standard NTSC fore, at for video calls for amplitude modulation of the video carder, means for converting FM signals to NTSC AM format are required either at the television set or at the point at which the fiber transmission trunk is connected to a coaxial distribution network. The need for such FM to NTSC AM conversion increases the cost of the system.
In view of the above, a system in which the video baseband signal amplitude modulates a radio frequency carrier signal which in turn amplitude modulates an optical signal is preferable to other systems from the standpoint of cost and simplicity. AM systems for long haul point-to-point delivery of high quality information television signals have been developed. As noted, such systems are more economical than digital or FM systems and have the benefit of maintaining the AM signal format throughout the system. Additionally, AM signal delivery is transparent to scrambling and compression techniques.
However, several phenomena limit the number of radio frequency channels which can be carded by present day optical links where the intensity of light signals is amplitude modulated. A first of these phenomena is a limitation of the amount of radio frequency energy which may be supplied as a modulating signal to a laser or other light generating device before various types of distortions are generated by the light generating device. This power limitation relates to the sum of the radio frequency power contributions of each radio frequency channel. Thus, if it is desired to transmit 80 radio frequency channels over a single optical link, each of these channels can be powered with only half of the power which would be available if only 40 channels were transmitted over the link. Such a limitation on the power of each radio frequency carrier brings each of these carried closer to the white noise level of the system, thus, adversely affecting the signal to noise ratio of the system. Decreasing the number of channels carried by each optical link in order to improve the signal to noise ratio increases the number of lasers and optical fibers which must be used, and thus the overall complexity and cost of the system. On the other hand, trying to increase the amount of radio frequency power supplied to the laser beyond certain limits causes the laser to produce several types of distortion, thereby degrading signal quality.